Abstract. We present a comparison of chemistry-transport models (TransCom-N2O) to
examine the importance of atmospheric transport and surface fluxes on the
variability of N2O mixing ratios in the troposphere. Six different
models and two model variants participated in the inter-comparison and
simulations were made for the period 2006 to 2009. In addition to N2O,
simulations of CFC-12 and SF6 were made by a subset of four of the
models to provide information on the models' proficiency in
stratosphere–troposphere exchange (STE) and meridional transport,
respectively. The same prior emissions were used by all models to restrict
differences among models to transport and chemistry alone. Four different
N2O flux scenarios totalling between 14 and 17 TgN yr−1 (for 2005)
globally were also compared. The modelled N2O mixing ratios were
assessed against observations from in situ stations, discrete air sampling
networks and aircraft. All models adequately captured the large-scale
patterns of N2O and the vertical gradient from the troposphere to the
stratosphere and most models also adequately captured the N2O
tropospheric growth rate. However, all models underestimated the
inter-hemispheric N2O gradient by at least 0.33 parts per billion (ppb),
equivalent to 1.5 TgN, which, even after accounting for an overestimate of
emissions in the Southern Ocean of circa 1.0 TgN, points to a likely
underestimate of the Northern Hemisphere source by up to 0.5 TgN and/or an
overestimate of STE in the Northern Hemisphere. Comparison with aircraft data
reveal that the models overestimate the amplitude of the N2O seasonal
cycle at Hawaii (21° N, 158° W) below circa 6000 m,
suggesting an overestimate of the importance of stratosphere to troposphere
transport in the lower troposphere at this latitude. In the Northern
Hemisphere, most of the models that provided CFC-12 simulations captured the
phase of the CFC-12, seasonal cycle, indicating a reasonable representation
of the timing of STE. However, for N2O all models simulated a too early
minimum by 2 to 3 months owing to errors in the seasonal cycle in the prior
soil emissions, which was not adequately represented by the terrestrial
biosphere model. In the Southern Hemisphere, most models failed to capture
the N2O and CFC-12 seasonality at Cape Grim, Tasmania, and all failed at
the South Pole, whereas for SF6, all models could capture the
seasonality at all sites, suggesting that there are large errors in modelled
vertical transport in high southern latitudes.